In the offshore industry there is a need for underwater pipelines. These underwater pipelines may be provided by joining pipe sections together to form a pipeline which is lowered from a vessel to an underwater bottom. In such a pipe laying method a plurality of pipe sections are welded to one another to form the pipeline, hang down from a vessel, where an upper end of the most recently joined pipe section is to be welded to a lower end of a new pipe section being lined up above said upper end. The pipe ends of the pipe sections may be provided with a collar for handling of the pipe sections and the pipe line. Upon completion of a weld, the end of the pipeline is gripped by the collar with a clamp and lowered to just below the welding position to prepare for a new pipe section to be arranged above the upper end of the lowered pipeline. Both pipe ends have pipe end preparations, so called bevels that are made to create the weld gutter when the pipe ends are placed on each other.
The welding of two pipe sections requires exact positioning of the welding equipment, in particular the welding heads with respect to the weld gutter. Also, the position of the two pipe ends to be joined relative to each other is subjected to strict requirements before welding may start. Since pipe sections from a pipe manufacturer may not be perfectly shaped, it is impossible to get a line up where the two pipe ends line up perfectly with each other all around the circumference of the pipe. At least at certain locations the walls of the pipe sections may not line up exactly with each other. The misalignment between the pipe walls of the two pipe sections is known as hi-lo. Around the circumference of the pipe the maximum hi-lo is generally kept as small as possible and should always be smaller than a maximum specified value, which may be as small as 0.5 millimeter. Small hi-lo values are required in particular when welds are subjected to fatigue loading as is for instance the case in risers. For fatigue loaded welds it is important to keep the bending stresses, and stress concentration over the weld as low as possible in order to achieve a favorable fatigue life of the weld. Positioning the pipe ends relative to each other is a delicate job that needs to be performed very accurately so that the hi-lo is minimized over the full circumference of the gutter. Currently the process of getting the best line-up is performed manually and can cost considerable time of adjustment and checking, especially when requirements are strict. Measurement systems according to the prior art that may assist in the line up are not able to measure all the geometrical data. Measurement systems according to the prior art (see for example WO2006/112689) are provided with a measuring device for measuring geometrical data in a measurement direction which is perpendicular to the centerline of the pipe. In this way, it is difficult if not impossible to measure the thickness of the landing zone and the hi-lo between the inner diameters of both pipe ends.